This invention relates to an apparatus for imparting desired amount of buoyancy to submerged articles, and more particularly to an apparatus which give buoyancy to such articles as pipe lines to be laid along the bottom of a body of water, thereby to reduce their weight in water and enable more efficient horizontal and vertical movements thereof.
One method of moving weighty items on the bottom of a body of water, or in the water, such as pipeline laying, is to build a working base on land, connecting the pipes at the base, and then successively drawing the connected pipes to the opposite shore through the water.
When the apparent weight in water of the article being drawn through the water or along the bottom is too light, the article may be moved by surges or currents in the water. The article must therefore be suitably weighted, such as by applying concrete to the outer surfaces thereof. However, conversely, if the apparent weight is too heavy, friction is caused between the article being drawn and the bottom of the water, thereby resulting in difficulties of drawing. In the latter case, means are provided throughout the length of the article being drawn to reduce the apparent weight thereof in water. When the drawing is completed, the buoyancy providing means are removed manually by divers.
One problem in, for example pipe laying, using prior art buoyancy means is in the nature and handling of the buoyancy means. When drawing the pipe line from one shore and along the bottom to the opposite shore, the water pressure changes according to the depth of the water. In order to suitably reduce the weight in water, the buoyancy means should maintain constant the buoyancy irrespective of the water depth or changes in water pressure. Also, on a practical basis, the buoyancy means should be easy to transport, store and attach to the submerged article.
Conventional buoyancy means have comprised floats made of steel pipes having both ends closed air-tight and attached to the pipeline, for example, to give suitable buoyancy thereto. The capacity of the floats having such stiff shells, may be kept constant in spite of the changes of water pressure. However, disadvantageously, the prior floats are expensive, and the weight and volume are large so that transportation expense is high and a large storage space is required. Moreover, it is troublesome to attach the prior floats to the pipeline being laid because of their heavy weight; it is necessary to use large scale loading machines, such as cranes. Also, disadvantageously, when removing the prior stiff shelled means from the article being drawn through the water, the floats tend to rapidly surface through the water due to their large buoyancy, thereby resulting in dangerous possibilities of colliding with machinery and workers.
Another prior buoyancy means uses floats of an air tight bag made of synthetic fibric into which air is filled. This prior means presents no problem in transportation or storage; however, disadvantageously, it is difficult to regulate the buoyancy to counteract changes in water depth. Thus, when this means is attached to the article being drawn, the buoyancy is changed adversely to cause either the article to rise or the article to be dragged by friction. This prior means cannot be used for water depths of more than 20 m.